Differences in seed morphometrics of representative orchids native to North America and Hawaii using scanning electron microscopy
Matthew H. Collier, Julianna S. Fisher, Kevin M. Gribbins, Jay A. Yoder, Lawrence W. Zettler
Abstract
Ultrastructure and dimensions of whole seed, embryo, and testa cells of 10 orchid species native to North America and Hawaii are reported. Seeds of seven terrestrials (Cleistes bifaria, Isotria medeoloides, Liparis elata, L. hawaiensis, Platanthera holochila, P. integrilabia, and P. leucophaea), two epiphytes (Encyclia tampensis, Epidendrum nocturnum), and one semi-aquatic (Habenaria repens) orchid were studied by light and scanning electron microscopy. In contrast to the epiphytes and the semi-aquatic orchid, seeds of terrestrial species were larger in size evidenced by a small length/width ratio, had greater seed volume, a larger number of testa cells, and embryos with the highest mean area, supporting the existing literature. Interestingly, seeds of both Hawaiian species differed noticeably from their mainland counterparts (L. hawaiensis vs. L. elata; P. holochila vs. P. integrilabia and P. leucophaea). Our study lends support for differences between genera, but also within at least two genera (Liparis, Platanthera), for orchids occupying different habitats separated by thousands of km (Hawaii, Florida). For orchids threatened with immediate extinction in North America and worldwide, a better understanding of seed morphometrics, and especially the structure and function of the testa, may be useful in developing more effective protocols aimed at in vitro seed germination.